1. Field of the Invention
The present invention relates to a light-scanning optical system and light-scanning apparatus using it and, particularly, is suitably applicable to image-forming apparatus, for example, such as laser beam printers, digital copiers, etc. involving the electrophotographic process, constructed to reflectively deflect light emitted from light source means (a single light source or plural light sources) by a polygon mirror as an optical deflector and optically scan a region on a surface to be scanned, through an f-xcex8 lens system having the f-xcex8 characteristics to record image information thereon.
2. Related Background Art
In the light-scanning optical systems (light-scanning apparatus) such as the laser beam printers and the like heretofore, the light emitted as optically modulated according to an image signal from the light source means is periodically deflected by the optical deflector, for example, consisting of a rotary polygon mirror (polygon-mirror), and the deflected light is converged in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by the f-xcex8 lens system having the f-xcex8 characteristics, to optically scan the region on the surface of the recording medium to effect image recording thereon.
FIG. 19 is a schematic diagram to show the major part of a conventional light-scanning optical system. In the same figure a diverging beam emitted from light source means 91 is converted into a nearly parallel beam or into a converging beam by a collimator lens 92. an aperture stop 93 shapes the beam (light amount), and the thus shaped beam is incident to a cylindrical lens 94 having its refracting power only in the sub-scanning direction. The beam entering the cylindrical lens 94 emerges in an as-incident state in the main scanning section while being converged in the sub-scanning section, thereby being focused as a substantially linear image near a deflection facet 95a of the optical deflector 95 consisting of a rotary polygon mirror (polygon mirror).
Then the beam reflectively deflected by the deflection facet 95a of the optical deflector 95 is guided through the f-xcex8 lens system 96 having the f-xcex8 characteristics onto a surface of photosensitive drum as a surface to be scanned 97, and the optical deflector 95 is rotated in the direction of arrow A to optically scan the region on the photosensitive drum surface 97 in the direction of arrow B (the main scanning direction), thereby recording image information thereon.
In order to implement highly accurate recording of image information in the image-forming apparatus using the light-scanning optical system of this type, it is necessary to meet the following requirements: the curvature of field is well corrected across the entire surface to be scanned, so as to equalize spot sizes; and the system has such distortion (f-xcex8 characteristics) as to establish the proportional relation between angles and image heights of the beam reflectively deflected by the optical deflector.
Meanwhile, there are demands for optical systems capable of scanning at high speed because of an increase in speed and definition of laser beam printers, digital copiers, and so on. Since there are limitations to the rotational speed of a motor, which is part of the scanning means, to the number of facets of the polygon mirror, which is part of the deflecting means, and so on, there are increasing desires, particularly, for multi-beam scanning optical systems capable of scanning the surface simultaneously with a plurality of beams emitted from a plurality of light-emitting regions (light sources).
In order to equalize the spot sizes throughout the entire surface to be scanned in such multi-beam scanning optical systems, the curvature of field needs to be well corrected for, while lateral magnifications in the sub-scanning direction need to be equalized throughout all the image heights. If the lateral magnifications in the sub-scanning direction differ depending upon image heights, spot sizes in the sub-scanning direction will vary depending upon image heights.
Unless the lateral magnifications in the sub-scanning direction are equalized throughout all the image heights, there will arise another problem that when the light-emitting regions are located off the optical axis in the sub-scanning direction as in the multi-beam scanning optical systems, scanning lines will be curved and the spacing between the lines in the sub-scanning direction will vary depending upon the image heights, thus causing degradation of image quality.
The various light-scanning optical systems for solving such problems have been proposed heretofore, for example, in Japanese Patent Application Laid-Open Nos. 8-297256, 10-232347, and so on.
The light-scanning optical system described in Japanese Patent Application Laid-Open No. 8-297256 is constructed in such structure that change in the F-number in the sub-scanning direction depending upon image heights of the beams incident to the surface to be scanned, is suppressed by continuously changing curvatures in the sub-scanning section of at least two lens surfaces of a lens constituting the f-xcex8 lens system from on the axis toward off the axis in the effective part of the lens. The above application describes the example in which the beams incident to the f-xcex8 lens system are converging beams and in which the change in the F-number in the sub-scanning direction is suppressed well.
The scanning optical system described in Japanese Patent Application Laid-Open No. 10-232347 is constructed in such structure that the two lenses constituting the f-xcex8 lens system are provided with an optimum combination of respective refracting powers in the sub-scanning section. The application describes the example in which the beams incident to the f-xcex8 lens system are converging beams and in which the spot sizes in the sub-scanning direction are equalized on the surface to be scanned.
An object of the present invention is to nullify occurrence of jitter and effectively correct an asymmetric component in the curve of scanning line occurring in use of relatively large polygon diameters and a total inclination component in the magnifications in the sub-scanning direction across the entire scanning area in a light-scanning optical system (or a multi-beam light-scanning optical system) or in light-scanning apparatus (or multi-beam light-scanning apparatus) wherein the beam incident to the f-xcex8 lens system is a nearly parallel beam, and thereby provide a light-scanning optical system capable of providing an image at high speed and with high quality while equalizing the spot sizes throughout the entire surface to be scanned and nullifying the curve of scanning line, and also provide light-scanning apparatus using it.
A light-scanning optical system according to one aspect of the invention is a light-scanning optical system comprising a first optical system for converting a beam emitted from light source means into a nearly parallel beam, a second optical system for focusing the converted beam into a linear beam along a main scanning direction on a deflection facet of deflecting means, and a third optical system for focusing the nearly parallel beam deflected by the deflecting means, on a surface to be scanned,
the light-scanning optical system being constructed so that an optical axis of an incidence optical system including the first optical system and the second optical system is inclined relative to a normal to the surface to be scanned, at least in the main scanning section,
wherein the third optical system comprises at least one optical element and the at least one optical element is arranged so that in the main scanning section a symmetry axis in the main scanning direction of the optical element is inclined relative to the normal to the surface to be scanned, so as to bring an end of the optical element on the light source means away from the deflecting means.
In one aspect of the above light-scanning optical system, the optical element arranged as inclined is a lens.
In a further aspect of the above light-scanning optical system, the optical element arranged as inclined is a reflecting mirror.
In a further aspect of the above light-scanning optical system, the optical element arranged as inclined includes a diffraction optical element.
In a further aspect of the above light-scanning optical system, the third optical system comprises two lenses.
In a further aspect of the above light-scanning optical system, a lens disposed on the surface-to-be-scanned side out of the two lenses of the third optical system has a larger refracting power in a sub-scanning section than that of the lens disposed on the deflecting means side.
In a further aspect of the above light-scanning optical system, the lens arranged as inclined is a lens disposed on the surface-to-be-scanned side out of the two lenses of the third optical system.
In a further aspect of the above light-scanning optical system, an optical axis of one lens out of the two lenses of the third optical system is arranged to be shifted by a predetermined distance in the main scanning direction, relative to an optical axis of the other lens.
In a further aspect of the above light-scanning optical system, the two lenses of said third optical system are first and second toric lenses arranged in the order named from the deflecting means side, the first toric lens has at least one lens surface of an aspherical shape in the main scanning section and is formed having a meniscus shape of a positive refracting power with a concave surface facing the deflecting means near a symmetry axis in the main scanning direction of the lens, and the second toric lens has two lens surfaces of an aspherical shape in the main scanning section and is formed in a meniscus shape having a positive, weak refracting power or almost no refracting power with a convex surface facing the deflecting means near a symmetry axis in the main scanning direction of the lens.
In a further aspect of the above light-scanning optical system, the first and second toric lenses in the sub-scanning section both have a meniscus shape with a concave surface facing the deflecting means.
In a further aspect of the above light-scanning optical system, curvatures of the two lens surfaces of the second toric lens in the main scanning section continuously vary from near the symmetry axis in the main scanning direction of the lens toward peripheral portions of the lens and signs of the curvatures are inverted in an intermediate portion.
In a further aspect of the above light-scanning optical system, the retracting power of said second toric lens in the sub-scanning section continuously varies on a symmetric basis from near the symmetry axis in the main scanning direction of the lens toward peripheral portions of the lens.
In a further aspect of the above light-scanning optical system, the refracting power of said first toric lens in the sub-scanning section continuously varies on an asymmetric basis from near the symmetry axis in the main scanning direction of the lens toward peripheral portions of the lens.
In a further aspect of the above light-scanning optical system, the refracting power of said first toric lens in the sub-scanning section becomes continuously stronger from near the symmetry axis in the main scanning direction of the lens toward peripheral portions of the lens and the refracting power of said second toric lens in the sub-scanning section becomes continuously weaker from near the symmetry axis in the main scanning direction of the lens toward peripheral portions of the lens.
In a further aspect of the above light-scanning optical system, the optical system satisfies the following condition:
|xcfx861s/xcfx862s|xe2x89xa60.1xe2x80x83xe2x80x83(Eq. 1),
where xcfx861s is the refracting power of the first toric lens in the sub-scanning section near the symmetry axis in the main scanning direction and xcfx862s is the refracting power of the second toric lens in the sub-scanning section near the symmetry axis in the main scanning direction.
In a further aspect of the above light-scanning optical system, the optical system satisfies the following condition:
|xcfx861m/xcfx862m|xe2x89xa7100xe2x80x83xe2x80x83(Eq. 2),
where xcfx861m is the refracting power of said first tonic lens in the main scanning section near the symmetry axis in the main scanning direction and xcfx862m is the refracting power of the second toric lens in the main scanning section near the symmetry axis in the main scanning direction.
In a further aspect of the above light-scanning optical system, the third optical system establishes a conjugate relation between the deflecting means and the surface to be scanned in the sub-scanning section and satisfies the following condition:
xe2x80x831.5xe2x89xa6xcex2cxe2x89xa64.0,
where xcex2c Is an image magnification in the sub-scanning section near the symmetry axis of the third optical system in the main scanning direction.
In a further aspect of the above light-scanning optical system, the third optical system establishes a conjugate relation between the deflecting means and the surface to be scanned in the sub-scanning section and satisfies the following condition:
0.9xe2x89xa6xcex2/xcex2cxe2x89xa61.1xe2x80x83xe2x80x83(Eq. 3).
where xcex2c is an image magnification in the sub-scanning section near the symmetry axis of the third optical system in the main scanning direction and xcex2 is an image magnification in the sub-scanning section at an arbitrary position off the axis in the main scanning direction.
In a further aspect of the above light-scanning optical system, the first toric lens and the second toric lens comprise a plastic material.
In a further aspect of the above light-scanning optical system, the light source means comprises a plurality of light-emitting points.
An image-forming apparatus according to another aspect of the present invention is an image-forming apparatus comprising the light-scanning optical system according to either one of the above aspects, a photosensitive member placed on the surface to be scanned, a developing unit for developing an electrostatic latent image formed on the photosensitive member by the beam by way of the light-scanning optical system, into a toner image, a transfer unit for transferring the toner image thus developed, onto a transfer medium, and a fixing unit for fixing the toner image thus transferred, on the transfer medium.
Another image-forming apparatus according to still another aspect of the present invention is an image-forming apparatus comprising the light-scanning optical system according to either one of the above aspects, and a printer controller for converting code data supplied from an external device, into an image signal and for supplying the image signal to the light-scanning optical system.